For several years the television producing industry has used an ingenious system for achieving composite images called the "chroma key system." One television camera senses a subject in front of a densely colored background, usually a deep blue hue. A second camera, sensing a second subject, delivers an image to a compositor or mixer that is "keyed " to accept or intensify the second signal only in those areas of the first camera image where the intense color background is apparent. The first color is normally not used in the second subject and image, since the compositor or mixer is keyed to reject electronically the keyed color.
Basically there are two systems utilized in the chroma key process. One is the "Demodulated Video Signal System" and the other is described as the "Hue-dial Signal System." Both are set forth in clear terms in a technical publication by Kennedy and Gaskins entitled "Electronic Composites in Modern Television."
As this article points out, the "Hue-dial Signal System" is more generally used because the difficult delay problems are nonexistent in the second delineated system, and the apparatus of the "Hue-dial Signal System" is simpler. However, in both systems there is no compensation for the visual disorientation caused by non-scale adjustment of one camera with respect to the other. One effect which is objectionable is when a "zoom" lens is used on the first subject and not on the second, whereby the effect is given of the first subject moving without volition or visible locomotion toward the viewer, destroying any illusion of reality. The instant invention overcomes this and other deficient areas of concurrent imagery conventionally practiced in the television art, and affords a flexible method applicable to making masters for film productions as well as for broadcast tapes.